In many
bird species, it's easy to tell males from females because the plumage
is different. Not so with eagles!

However,
we can use size measurements from different parts of the eagle's body
to determine the gender of the eagles we capture. Female eagles generally
have larger wings, feet, talons, beak, etc.

A biologist
by the name of Bortolotti discovered that 2 size measurements show
the greatest separation between male and female. The two measurements
are:

beak
depth and

hallux
(toe claw) length

These
measurements can be used in the following formula:

Sex
= (bill depth x 0.392) + (hallux length x 0.340) -27.694

If the
answer is POSITIVE, the eagle is a FEMALE.

If the
answer is NEGATIVE, the eagle is a MALE.

Here are
the key measurements from 6 of our eagles, plus their weights. Can
you determine the gender of these birds yourself? (Click
here for student worksheet.)

Eagle

Weight
(kg)

Bill
Depth (mm)

Hallux
Length (mm)

Gender

K58

5.5

37.9

44.5

K72

6.6

35.9

42.7

E47

6.7

35.0

42.2

E49

6.1

35.5

42.7

E63

4.5

32.6

40.5

E50

5.1

32.4

41.2

Challenges
of Using Weight to Determine Gender
We can also use weight to try to tell males and females apart--but when we talk
of bald eagle weights, we need to keep two factors in mind:

#1 Female
eagles are larger than the male eagles. On the average, females are
about 1/3 larger than males. (However, this only holds true if the
birds are in the same population. See #2.)

#2 Animals
living farther north are generally larger and heavier than those
living farther south.

This second
factor, called "Bergman and Allens rule," makes things a
bit more complicated:Bald eagles living farther north (in Canada
or Alaska, for example) should be larger than those say, from Florida.
And they are!

However,
there is some overlap. There is not always a definitive, clear separation
of the sexes based upon size and weight. For example, the largest northern
male eagle may be as big as or even bigger than the smallest southern
female. Understand?

Now, with that as some background, here are some sample eagle weights:

Body
weights over 7 kg are common. For example, 5 eagles captured in Alaska
recently weighed an average of 7.3 kg; the largest weighed 7.5 kg.

In contrast,
some southern eagles have weighed in at as little as 3 kg--less than
half the weight of some larger birds.

Generally
speaking, here in NY, we find male eagle weighing in from about 3.4-5.0
kg, while females are found within the 4.0-7.0 kg range.

By the way,
when the female of a species is larger than the male it is known as "reverse
sexual dimorphism." This is true in most raptors, including the
bald eagle. (In most species the opposite is true. When males are larger,
it's known as "sexual dimorphism.")

Journaling
Questions

What
other animal species can you find that shows reverse sexual dimorphism?

Female
eagles found in NY generally are within the 4.0-7.0 kg range. Males
are in the 3.4-5.0 kg range. What kinds of things in your classroom
weigh about the same as these birds? (hint:Convert these numbers
to pounds if that will be easier to imagine?) Brainstorm a list and
then weigh on the scale in the nurses' office.